1. Field of the Invention
The present invention relates to an electron beam exposure apparatus and electron beam exposure method, more particularly to an improvement of function and method for correcting proximity effect when exposing pattern of semiconductor circuit of large scale integration (referred to as LSI hereinafter).
2. Description of the Related Art
Recently, as the integration density of LSI devices has been increased, there is required technology which may provide the exposure of fine pattern of less than 0.1 .mu.m at high resolution. For example, when forming a device pattern onto a semiconductor wafer, photo-transfer technology with ultraviolet light is used.
In accordance with the photo-transfer technology mentioned above, a mask or a reticle made of an original drawing magnified about 5 times is used for a master. In the mask forming process of this case, a dry plate is formed first. A dry plate is made by a glass substrate, on which metal layer is spattered or deposited for opaque layer. Then resist for electron beam is applied thereon, and the substrate is baked. The plate is selectively radiated with electron beam and etched. Then additional processes such as peeling off residual resist are done. Thus, a mask (reticle) is formed by selectively removing metal layer from the substrate.
However, in the electron beam exposure, the emitted electrons penetrating into the dry plate may diffuse and disperse within the plate. This leads to the phenomenon called "Proximity effect". The proximity effect is caused by the influence of the amount of effective radiation to the area of higher pattern density, as compared with the amount of effective radiation to the independent pattern area within the radiated zone. For example, the proximity effect is caused by the radiated beam of each of the shots superposed on each other in that area.
At this point, the related arts of the present invention will be described. For example, as depicted in FIG. 1, an apparatus for carrying out "GHOST exposure method" for correcting the proximity effect, comprises a CPU 1, terminal unit 2, magnetic disk 3, interface 4, buffer memory 5, beam control system 6, register for dose 7, stage driving system 8, and Electron beam optics 9.
In the operation of said apparatus, when at first an exposure instruction is entered through terminal unit 2, main exposure data Dm and sub-exposure data Ds are read out from magnetic disk 3, these data are stored in the buffer memory 5 via interface 4. Then, data Dm which determines the amount of dose Qe in the most appropriate focus (just focus) is set to register 7, and the exposure is done in accordance with this data. After having exposed with this data Dm (simply referred to as Dm exposure, hereinafter), another dose amount Qc for the sub-exposure pattern Ds is set to register 7 to make another exposure with data Ds (hereinafter, Ds exposure).
By this, LSI pattern is exposed onto the exposed workpiece 15 according to the main exposure data Dm and sub-exposure data Ds. The exposure with sub-exposure data Ds is used for correcting the proximity effect.
The principle of this method is to form a reversal image ("back ground") of the main exposure data ("primary pattern"). That is, supplying to the auxiliary pattern as shown in FIG. 2A (height H.times.width W) electron beam EB in which the dose amount of from 30 to 40% of the dose amount of the main exposure is shade off by a predetermined amount. More practically, adjusting the coil current which is supplied to the object lens 9A of the electron beam optics 9 to defocus, thereby maintaining the energy for the back ground to a given level to obtain a desired and uniform pattern size.
According to a paper entitled "proximity effect correction for electron beam lithography by equalization of background dose" by G. Owen and P. Rissman, appeared on J. Appl. Phys., 54, 3573 (1983), following equation is given: EQU Qc=Qe.times..eta.e(1+.eta..sub.e).sup.-1, EQU dc=2.sigma..sub.b .times.(1+.eta..sub.e).sup.-1/4
Where .eta..sub.e is the ratio between the energy stored in the resist layer by backward diffused electrons and that by front diffused electrons; Qe is the amount of dose for pattern exposure; Qc is the amount of dose for supplying to the reversal image area of the pattern in order to correct the proximity effect; .sigma..sub.b is the radius at which the energy stored in the resist layer by the backward diffused electrons gets 1/e of its peak value (e=Euler constant); dc is the shaded beam radius at the moment of exposure of the reversal image area for correction.
In the above-mentioned paper, a set of values of .eta..sub.e =0.73, Qc/Qe=0.43, and dc=3.2 .mu.m is cited as an example.